1. Field
Some embodiments are related to light emitting devices. Some embodiments are related to elements for enhancing light extraction from light emitting.
2. Description of the Related Art
Solid state light emitting devices such as light emitting diode (LED), organic light emitting diode (OLED) or sometimes called organic electroluminescent device (OEL), and inorganic electroluminescent device (IEL) have been widely utilized for various application such as flat panel display, indicator for various instrument, signboard, and ornamental illumination, etc. As the emission efficiency of these light emitting devices continues to improve, applications that require much higher luminance intensity, such as automobile headlights and general lighting, may soon become feasible. For these applications, white-LED is one of the promising candidates and have attracted much attention.
Conventional white-LED have been manufactured based on the combination of blue-LED and yellow light-emitting YAG phosphor powder dispersed in plastic encapsulant resin like epoxy and silicone as disclosed in U.S. Pat. No. 5,998,925 and U.S. Pat. No. 6,069,440. However, since the particle size of YAG phosphor powder utilized for this system is around 1-10 μm, the YAG powders dispersed in the encapsulant resin medium can cause strong light scattering. Consequently, a considerable portion of both the incident light from the blue LED and the yellow emitting light from YAG powders ends up being back scattered and dissipated as a loss of white light emission.
One response to this problem is to form a monolithic ceramic member of wavelength converting material. The ceramic member can be plural ceramic layers of single or multiple phosphors, or transparent layers. The transparent ceramic layers may be, for example, the same host material as the activating dopant. (U.S. Pat. No. 7,361,938). These laminated layers may also be in the form of luminescent ceramic cast tapes, which can be laminated and co-fired with the host material. (U.S. Pat. No. 7,514,721 and US Published Application 2009/0108507).
For phosphor ceramic, ultrahigh purity of the host material is desired to provide high wavelength conversion efficiency. Such phosphor ceramics can be processed through solid-state reaction. Transparent phosphor ceramic plate can be applied to obtain white LED with combining with blue LED. However, transparent phosphor ceramic plate contains emitted yellow light within the ceramic plate, due to high refractive index (˜1.83). As a result, the efficacy of the white LED cannot be maximized without better light out-coupling. Thus there is a need for a ceramic element that improves the light out-coupling leading to higher efficacy of white LED device.